This invention relates to the operation of electrochemical cells and particularly to the operation of electrochemical cells containing an aqueous electrolyte in a supercritical fluid (SCF) state.
The liquid-vapor critical temperature (Tc) of a pure substance is the temperature at which, in a closed container, the vapor phase reaches the same (critical) density as the liquid phase. With rising temperature at vapor saturation, the two phases merge into a single supercritical fluid at this temperature, which is evidenced visually by the disappearance of the meniscus separating the two phases. Accordingly, there also is a unique critical temperature, critical pressure, and other critical properties for each compound. Solutions exhibit the same phenomena whereby the composition as well as the density of the two phases becomes identical at the critical temperature of the solution.
For example, to bring water to the supercritical state, the temperature and pressure must be raised above 374.degree. C. and 22.06 mega Pascals (MPa), respectively. In the case of supercritical water (SCW), the density, dielectric constant, hydrogen bonding, and certain other physical properties are so altered that water behaves much as a moderately polar organic liquid would under ambient conditions. SCW, above 374.degree. C., is miscible with gases such as nitrogen, oxygen and air in all proportions.
An excellent discussion of SCF's can be found in "Supercritical Fluids", Environmental Science and Technology, Volume 16, No. 10, pp. 548A-551A.
Electrochemical cells are used throughout industry for a variety of purposes including electrolytic production of compounds, electroplating, and as batteries or fuel cells.
To minimize the amount of energy consumed, it is desirable to operate an electrochemical cell in the most efficient manner possible. One way of doing so is to maximize the mass transfer rates of reactants and products to, and from, the surface of the electrodes. This can be accomplished, for example, through the use of turbulence promoters located inside the electrolysis compartments of cells. Turbulence promoters take the form of nets, inert beads, strips, or rods, perpendicular to the flow the electrolyte or they may be forms machined on the electrode itself. Turbulence promoters are designed to increase the mass transfer rates of materials to, and from, the electrode surfaces, but provide only marginal improvement.
It would be desirable to increase the efficiency of electrochemical cells without the need for turbulence promoters or other devices. The present invention provides such a method.